Bleaching plant and method of bleaching cellulose pulp



2,711,359 BLEACHING PLANT AND METHOD OF BLEACHING CELLULOSE PULP FiledOct. 19, 1953 June 21, 1955 J. JOHANSEN 3 Sheets-Sheet 1 I 7 4 r 5 7 2 a4 w "T g m w I J m u n .Q H X INVENTOR.

/?o/f Jo mar do/mmsen BYM Mr- M i June 21, 1955 v J. JOHANSEN 2,711,359

BLEACHING PLANT AND METHOD OF BLEACHING CELLULOSE PULP Filed Oct. 19,1955 3 Sheets-Sheet 2' June 21, 1955 R- J. JOHANSEN 2,711,359

BLEACHING PLANT AND METHOD OF BLEACHING CELLULOSE PULP Filed Oct. 19,1955 3 sheets-sheet 5- IA/ VE N TOR Rolf Jamar- Jo/z nae/7, BY l vATTORNEYS.

limited States Patent BLEACHING PLANT AND METHOD OF BLEACHTNG CELLULQSEPULP Rolf Jamar .lohansen, Karlstad, Sweden, assignor to AktieholagetKamyr, Karlstad, Sweden, a company of Sweden Application Gets-her 19,1953, Serial No. 386,960

Claims. (Cl. 8-156) being followed by a filtration operation in whichthe I retained pulp is washed. The height of the reaction tower isusually of the order of to meters, but in exceptional cases the towermay be as short as 10 meters or as long as meters. The great dillerencesin height between the upper and lower ends of the tower present thepossibility of producing the pressure difference between the inlet andthe outlet side of the filter necessary for the operation of the same,without the use of pumps. ireviously, suction or vacuum filters havebeen used, in which the necessary vacuum has been derived from a suctionor drop tube in which filtrate water is allowed to drop 8 to 10 meterscarrying air with it. In this way a vacuum corresponding to a watercolumn of 1 to 3 meters is produced in the filter. In view of thepresence of said drop tube the filters and the attendance floor areusually placed at a level of about 15 to 25 meters above the ground, theroof height of the enclosing building being 20 to 30 meters.

An object of the present invention is to achieve an improvement and asimplification of bleaching plants,

especially aimed at reducing the volume of the building, decreasing thebuilding costs and effecting a better localization of the filterattendance floor in relation to neighboring departments, such asscreening and beating departments, drying machines etc.

Other and further objects will be apparent from the description of theinvention which follows.

According to the invention, these objects are attained by supplying thepulp containing liquid, together with air to the filter undersuper-atmospheric pressure and employing the height difference betweenthe top of the reaction tower and the ground to create the aforesaid.

pressure instead of creating a vacuum as has heretofore been common inbleaching plants. The filter may consequently be placed at a low level,i. e., ground level or only slightly thereover to thereby achieve asimplification of the structure necessary to house the equipment and aconsiderable reduction in building costs.

In accordance with the invention, a drop tube, preferably directlyconnected to the top of the reaction tower, is interposed between thebleaching operation and the filtration operation. The upper end of thedrop tube is permitted to have free access to air, so that the pulpcontaining liquor dropping down therein carries air with it to createthe pressure diiferential necessary for the operation of the filter in asimple and effective manner. The lower end of the drop tube is connectedto an air separating device, preferably of cyclone type. The air outletof the separating device is connected to the air' inlet of the pressurefilter, and the pulp outlet of the separating device is connected to thepulp inlet of the filter.

When the plant comprises a plurality of bleaching operations constructedaccording to the invention, the

lice air outlets of the several separating devices may be connectedtogetherf The pressure air conduit connected to one or moreairseparating devices may be branched oil to the pressure air inletof apressure filter not equipped with a separate drop tube for the supply ofpulp containing liquor and air under pressure to thereby avoid the useof a compressor. Said conduit may'also be connected to a compressorserving as a reserve source of air under pressure which can be used whenstarting up the filters or when disturbances occur in the plant.

By placing the filters at a comparatively low level in accordance withthe invention it is possible to radically change the shape of thebuilding in which the bleaching plant is housed. Thus, the filters maybe placed in a low building and put either approximately on the groundlevel or preferably on an attendance floor arranged a'few meters abovesaid level, whereas the reaction towers which usually are made ofconcrete, are completelyor partly placed in open air; The towers mayeither project up through the roof of the building which houses thefilters and auxiliary apparatus, or be placed at the side of saidbuilding. In order to protect the discharging devices located at the topof the reaction towers, these towers may be provided with protection inthe form of a hood or similar covering structure. towers are placedclose to each other, the hoods may be built together to form a unitarysuperstructure.

The invention is'illustrated by the followingdescription in whichreference is made to the accompanying drawings wherein:

Figure 1 shows diagrammatically and partly in section an elevationalview of a bleaching plant constructed in accordance with theinvention,

Figure 2 is an elevation on an enlarged scale and partly in sectionshowing the details of the air-pulp containing liquor separating device,the pressure filter and associated parts, a v i Figure 3 is a plan viewof Figure 2,

Figure 4 is an elevation plant constructed according to the inventionshowing the building therefor,- and Figure 5 is a cross-sectional viewtaken substantially on the lines 55 of Figure 4. I Referringto Figure 1,11 designates a high cylindrical concrete tower in which cellulose pulpis treated with a bleaching agent, such as chlorine, hypochlorite ors'ul-- phur-dioxide. ,The pulp'is supplied through a conduit 13 at thebottom of the tower and moves, mixed with the 1 bleaching agent, slowlyupwards in the same, so that the desired bleaching reaction is completedwhen thepulp reaches the top ,of the tower. There the pulp isdilutedwith diluting water taken from the conduit 15, preferably to a drycontent of 1 to 3 per cent.

the pulp falling through the drop tube 2 3 will carry with essentiallyvertical drop tube 23 15 meters or more and across regard to the pulpquantity acprincipals which apply to drop it air which, due to the highdrop velocity, cannot pass I liquor in a direction coun back past thepulp containing ter-current thereto butis compressed and carried alongwith the pulp to a separating device 25 connected to the lower end ofthe drop tube.

If a plurality of partly in section of a bleaching By means of a rotaryscraping device 17 (which may be omitted or 'replaced by a feeding screwor similarequipment) the pulp Arranged between the outlet 19' theatmosphere, and therefore,

Preferably, the separating device 25 consists of a cylindrical vesselinto which the tube 23 enters tangentially, whereby pulp and air areseparated by cyclonic action. Connected to the bottom of said vessel. isa conduit 27 leading to the pulp inlet 29 of the filter 21. Provided onthe cap of the vessel 25 is an air outlet 31 which is connected by aconduit 33 to the air inlet 35 of the filter.

The pressure filter 21 consists of a closed cover 37 and a sieve drum 39mounted for rotation therein. The cover is sealed to the ends of thesieve drum so that superatmospheric pressure can be maintained in thespace between the cover and the sieve drum. In operation of the filter,the space between the cover and the sieve drum is partly filled withpulp containing liquor introduced into the lower portion of the coverthrough the inlet 27 and partly filled with air under pressureintroduced near the top through the air inlet 35. The interior of thesieve drum is maintained under atmospheric pressure. Adjoining one endof the sieve drum is a stationary box 41 in which the filtrate passingthrough the sieve drum is collected. 43 designates sprinkle tubes bywhich the pulp layer formed on the screen surface of the drum issprinkled with washing liquid. The pulp layer is removed from the drumand taken out of the filter by means of a discharge roll 45. The airpressure within the space between the cover and the sieve drum ismaintained by the presence of the pulp containing liquor at the lowerportion thereof which acts as a seal, the close fit between thedischarge roll 45 and the sieve drum 39 and the layer of pulp on thesurface of the sieve drum. The pulp containing liquor is prevented fromescaping through the opening in the cover located below the dischargeroll 45 by a sealing strip which bears against the surface of the sievedrum. The filtrate collected in the box 41 runs in part into the sewer46, and another part thereof is by means of the pump 47 returned throughthe conduit to the top of the bleaching tower where it is used fordilution of the pulp leaving the tower.

Pulp and air are fed from the cyclone to the filter 21 under a pressurewhich is dependent upon the length and cross-sectional area of the droptube 23, upon the thickness and character of the pulp layer formed onthe filter surface, and upon the wash Water quantity etc. The actualoperating pressure may vary between values corresponding to water headsof about 1 to 5 meters. The filter is self-controlling in the respectthat when starting the same and when the pulp layer on the surface ofthe sieve drum is thin and highly pervious to air, the pressure of thepulp as well as of the air will be comparatively low, and the pulpdropping down in the drop tube will, on account of the reducedcounter-pressure in the lower end of said tube, carry with it acomparatively large volume of air, which is exactly what is required inthis case. When the pulp layer on the sieve drum becomes thicker andtighter, less air is required and the pressure in the filter rises.Because of the high back pressure so developed, the pulp in the droptube 23 can not carry with it as much air as previously, but instead theair that is entrained is compressed to a higher pressure. In order to beable to control the air quantity in relation to the pulp quantity avalve 49 is provided by means of which pressure air may be let otf. Saidvalve may be controlled automatically, e. g., by the pressure of thefilter, the pulp level therein or by other conditions of operation.

The pulp layer discharged from the filter 21 drops into a mixer 51 wherethe pulp is diluted with white water supplied through a conduit 53(shown in Figure l) and from where it is fed through a conduit 55 to ascreening device 57 (indicated generally in Figure 1). With reference toFigure l, the pulp is pumped from the screening device by means of thepump 59 to another pressure filter 61. The air under pressure necessaryfor the operation of this filter is taken from the conduit 33 connectedto the cyclone 25. Consequently no compressor is needed for supplyingair to this filter, although the same is not equipped with a separatedrop tube. However, for reserve purposes, it is preferable to have acompressor 63 which, when no longer needed, may be shut off by the valve65. If desired, the pressure air conduit connecting the two filters maybe closed by a valve 67, so that the filters will have separate airsupply sources.

The filtrate obtained in the filter 61 is collected in the box 69. Partof the filtrate goes into the sewer 71 and another part is pumped backby means of the pump 73 to the mixer l and is there used as dilutingwater. The dewatered pulp layer goes into a pulp bin 75 from where thepulp is pumped on to a drying machine or otherwise further handled inconventional manner.

The above described bleaching step consisting of the reaction tower 11and the pressure filter 21 may be the last step of a bleaching plant inwhich the pulp is treated in a number of steps by dilferent bleachingagents. In that case, the proceeding steps are constructed similarly.

Figures 4 and 5 show a bleaching plant consisting of three bleachingoperations each comprising a reaction tower 3.1, 11 or 11" and apressure filter 21', 21" or 2i cooperating in the above describedmanner, the building arrangements and the mutual location of the variousdevices being clearly shown therein. The three reaction towers areplaced in a row close to each other, and their lower parts are enclosedin a building 77 which is of considerably lower height than the towers,and through the roof of which the towers extend upwardly. Arranged insaid building at a height of a few meters above the ground level is afloor 79 on which the three pressure filters areplaced, each oppositethe tower with which it cooperated. The upper floor of the building alsohouses the air separating devices 25', 25", and 25". Placed on the lowerlevel are the pumps 47, 47 by means of which the pulp coming from thefilter of a preceding bleaching step is pumped into the bottom of thefollowing bleaching tower. Arranged around the tower tops is a commonsuperstructure 81 which covers and protects the upper ends of the towersand makes possible a comfortable supervision and attendance to thedischarge scrapers there. Between the superstructure 81 and the roof ofthe building 77 the bleach towers are not surrounded by a building ofany kind, which considerably reduced the volume and the cost of thebuilding.

It is not essential to the invention that the pulp supplied to thefilter through a drop tube is directly connected to the top of a highbleach tower. For example, the plant shown in Figure 1 may be modifiedby providing a vertical drop tube before the filter 61, the upper end ofwhich is connected to a box located at a suificiently high level. Thepulp can then be pumped up into said box by the pump 59, and when thepulp drops through the drop tube air is carried along and is compressed.This method of getting pulp as well as air under pressure by means of asingle pump may also be used for other purposes than for washing after ableaching operation, viz. for instance for washing off black liquorafter a digesting alkaline cook.

The present invention is illustrated by the following specific example;

In a bleach plant dimensioned for a capacity of 300 tons of pulp a day,digested ligno-cellulosic pulp having a consistency of 10% is bleachedin a bleach tower 11 having a height of 70 feet and a diameter of l6feet. The pulp suspension discharged at the top of the tower is dilutedwith 4000 gallons water per minute supplied through the tube 1.5,whereby the consistency of the pulp suspension is decreased to a drycontent of 1.5%. The drop tube 23 has a length of 40 feet and a diameterof l4 inches. The separating device 25 has a diameter of 8 feet and aheight of 8 feet. The filter drum 39 has a iameter of 10 feet and alength of 14 feet. The filter cover 37 has a diameter of 12 feet and alength of 14 feet. The air pressure generated in the device 25 is about12 inches Hg.

I claim:

1. In a process for bleaching cellulosic pulp containing at least onebleaching operation conducted in a reaction tower wherein the bleachedpulp is discharged at the top of the tower and thereafter washed in afilter, the improvement which comprises diluting the bleached pulp atthe top of the tower with a the diluted bleached pulp into asubstantially vertical drop tube the top of which has access to theatmosphere, discharging said diluted bleached pulp'together with the airunder pressure entrained therein from the lower end of said drop tubeinto a device for separating liquid and air, feeding said dilutedbleached pulp and said air under pressure into a pressure filter andremoving said pulp from the air and liquid present in said filter.

2. A process as recited in claim 1 in which the vertical distancebetween the top of said tower and said separating device is at leastapproximately 6 meters.

3. An apparatus for filtering pulp containing liquors comprising arotary drum filter having inlets for pulp containing liquor and airrespectively and capable of filtering said pulp from a liquor containingthe same supplied under pressure in the presence of air under pressure,a substantially vertical drop tube having its upper end open to theatmosphere, means communicating with said upper end for introducing pulpinto said drop tube whereby falling pulp containing liquor will entrainand compress air, the lower end of said drop tube communicating with adevice for separating liquid and air, said separating device havingconduits for leading the separated liquor and gas to the said inlets forliquor and gas respectively in said filter.

wash liquor, dropping 4. An apparatus as recited in claim 3 in whichsaid liquid-air separating device is a cyclone separator, the dischargeof said pulp containing liquor together with air under pressureentrained therein from said drop tube being discharged tangentially intosaid cyclone separator, said cyclone separator having a liquid ,outletat the bottom thereof connected to an'inletfor pulp containing liquorpositioned at a lower portion of said filter and;

having an air outlet at the top connected to an inlet for airf portionof said filter.

,5. In combination, having discharge means for liquid positioned at theupper end thereof, a device for separating liquid and air asubstantially vertical drop tube having its upper end open to theatmosphere, said upper end of said drop tube being positioned to receivethe liquid discharged from the upper end of said tower, a lower endcommunicating with said device for separating liquid and air, the lattersaid separating'device having of the separating device positioned atanupper a liquid outlet communicating with a lower portion of a r V rotarydrum pressure filter and an outlet for air communicating with an upperportion of said filter.

ReferencesCited in the file of this patent UNITED STATES PATENTS1,853,849 De Cew Apr. 12, 1932 1,915,812 Wollenberg June 27, 19332,138,143 Dodge Nov. 29, 1938 2,383,684 Richter Aug. 28, 1945 2,431,478Hill Nov. 25, 1947 2,571,219 De Cew Oct. 16, 1951 7 Simpson Jan. 1,1952

a reaction tower for treating pulp said drop tube having-

1. IN A PROCESS FOR BLEACHING CELLULOSIC PULP CONTAINING AT LEAST ONEBLEACHING OPERATION CONDUCTED IN A REACTION TOWER WHEREIN THE BLEACHEDPULP IS DISCHARGED AT THE TOP OF THE TOWER AND THEREAFTER WASHED IN AFILTER, THE IMPROVEMENT WHICH COMPRISES DIILUTING THE BLEACHED PULP ATTHE TOP OF THE TOWER WITH A WASH LIQUOR, DROPPING THE DILUTED BLEACHEDPULP INTO A SUBSTANTIALLY VERTICAL DROP TUBE THE TOP OF WHICH HAS ACCESSTO THE ATMOSPHERE, DISCHARGING SAID DILUTE BLEACHED PULP TOGETHER WITHTHE AIR UNDER PRESSURE ENTRAINED THEREIN FROM THE LOWER END OF SAID DROPTUBE INTO A DEVICE FOR SEPARATING LIQUID AND AIR, FEEDING SAID DILUTEBLEACHED PULP AND SAID AIR UNDER PRESSURE INTO A PRESSURE FILTER ANDREMOVING SAID PULP FROM THE AIR AND LIQUID PRESENT IN SAID FILTER.